EFFECTS OF EXOTIC EARTHWORMS (EISENIA FETIDA) ON THE RELEASE OF DISSOLVED ORGANIC CARBON FROM HEMLOCK AND DECIDUOUS FOREST SOILS OF WESTERN MASSACHUSETTS

The invasion of exotic earthworm species into forested soils in Minnesota has been shown to dramatically alter carbon and nitrogen cycling that can lead to an increase in the export of Dissolved Organic Carbon (DOC). In addition, an increase in the decomposition rate could also lead to the release of organically bound soil mercury accumulated from atmospheric deposition.

To evaluate the potential effects of invasive worms on forested watersheds in New England, a series of leaching experiments were done using O horizon soil samples collected beneath both deciduous and hemlock canopies within the Avery Brook watershed located in West Whately, Massachusetts. In this experiment an equal mass of a popular composting worm “Red Wiggler” (Eisenia fetida), was added to four replicates of each soil held in open plastic containers. These, along with an equal number of control samples, were watered weekly and the leachate was collected for analysis of DOC, SUVA, and total mercury. Prior to treatment, analysis of the organic horizons revealed mercury concentrations of approximately 200 ppb under the deciduous canopy and 150 ppb under the hemlocks.

The results of the experiment show that worms significantly increase organic decomposition rates. Concentrations of DOC in the leachate increased to over 300 mg/L in the worm bearing deciduous organic horizons compared to less than 100 mg/L in the control samples. Worm decomposition of the hemlock O horizons produced less DOC, with a maximum concentration of a little over 200 mg/L but still double what was observed in the controls. During the course of the experiment, Specific Ultraviolet Absorption (SUVA) decreased rapidly in worm bearing deciduous soils while the decrease occurred much more slowly in the hemlock soils. SUVA is a measure of the aromaticity of the DOC and the decrease in the samples with worms indicates more complete organic decomposition. Preliminary measurements of aqueous Hg_T show leachate concentrations near 200 ng/L for deciduous O horizons. In addition, the worm bearing soils all produced more drainage water indicating an even higher DOC and Hg_T flux. From this experiment it is clear that invasive worms have the potential to greatly increase DOC and Hg_T export from forested watersheds.